… or do they just write it down on a slip of paper, and reveal it when someone says it?

A: Yes, the player has to TELL the other players what the forbidden word is. If we wanted you to keep it hidden from the other players, write it down, reveal it at the moment invoked, we definitely would have put that in the instructions very explicitly. We feel that would be rather harsh to play it without telling people what they’re not allowed to say — more punitive and less fun, by far.

We feel the fun lies not in suddenly siccing it on some unsuspecting player, but in watching everyone speak in circles trying to avoid the word you’ve designated, and catching them when they’ve messed up… and know it.

Feel free to pick a ridiculously common word, however!
Suggestions: “you” “your” “turn” “card” “it’s” “the”… you get the picture…